Mold apparatus

ABSTRACT

A mold apparatus for forming a molded article comprising a hollow body of a crosslinked polymer rotationally molded around a section of at least one piece of non-crosslinked conduit, which conduits protrude from said hollow body, made by attaching said conduits of non-crosslinked material at the desired location to the mold with said section being located inside a heated section of the mold so that the crosslinkable polymer during a rotational molding process is fused and crosslinked inside of the mold and around and in tight welding-type connection with said section, whereas the remainder of the piece or pieces of uncrosslinked polymer are heat-insulated, thus avoiding deformation or collapsing of these pieces.

This is a divisional application of our copending patent applicationSer. No. 699,829, filed June 25, 1976, now U.S. Pat. No. 4,076,282,which in turn was a divisional application of Ser. No. 599,738, filedJuly 28, 1975, now U.S. Pat. No. 3,089,787.

This invention relates to plastic articles. In accordance with oneaspect, this invention relates to plastic pipe fittings and plastic pipeconnections. In accordance with another aspect, this invention relatesto methods for making such plastic articles. In accordance with afurther aspect, this invention relates to a new mold.

BACKGROUND OF THE INVENTION

Articles with intricate shapes can be formed out of thermoplasticpolymer materials. If such articles are to be connected to other units,they have been made out of non-crosslinked plastic polymers and sucharticles could easily be, for instance, welded to another unit. When ahigher degree of mechanical stability or rigidity was desired, sucharticles were formed out of crosslinkable plastic polymer which wascrosslinked during the molding step.

One process which is particularly advantageous for molding intricatelyshaped articles out of crosslinkable polymers is the rotational moldingprocess. In this process, powder or granules of plastic polymer beingcrosslinkable and containing a crosslinking agent are introduced into amold. The closed mold is then heated from the outside and rotatedpreferably around two axes with different rotational speeds. During thisprocess the polymer fuses and crosslinks, thus forming a layer ofcrosslinked material on the inner surface of the mold. Sturdy articleswith intricate shapes can economically be produced by this process.However, pipe connections or pipe fittings of crosslinked polymers havethe serious drawback that they cannot be connected by standardtechniques, e.g., welding or adhesive bonding, to other units such aspipes of non-crosslinked polymers or to other crosslinked sections.

THE INVENTION

An object of this invention is to provide a new mold for the formationof a novel plastic article.

These and other objects, features, advantages, and embodiments of thisinvention will become apparent to a person skilled in the art from thefollowing detailed description of the invention, the attached claims,and the drawing, in which:

FIG. 1 shows a perspective view of an open mold with a finished pipe teetherein,

FIG. 2 shows a cross-section through the mold shown in FIG. 1 along theplane separating the two mold halves at the end of a rotational moldingprocess, and

FIG. 3 shows a section of a mold with a butt-type joint incross-section, as explained in connection with FIG. 2 above.

In accordance with this invention, for a novel mold there is nowprovided a molded article of a polymeric material, which articleconsists essentially of a hollow body of crosslinked plastic polymerhaving bonded thereto at least one conduit of uncrosslinked plasticpolymer material. The term "bonded" means a tight, welding-typeconnection between the non-crosslinked conduit and the crosslinkedhollow body; the bond is made by rotationally molding the hollow bodyfrom crosslinkable plastic material around one end of each conduit ofnon-crosslinked material; the crosslinkable plastic material fuses andcrosslinks around this end of the conduit producing said tight,welding-type connection.

The article made in the mold of this invention has the advantage ofhaving the strength of the crosslinked polymer in the body section andof being readily connectable by the conduit or conduits of uncrosslinkedplastic material employing standard procedures such as welding oradhesive bonding.

The presently preferred plastic article is a pipe fitting having ahollow body of crosslinked polymer, said body opening into at least twopieces of pipe of non-crosslinked plastic polymer. This pipe-connectingpiece or pipe fitting can readily be connected to standard plasticpipes, e.g., sewer pipes, by welding the plastic non-crosslinked piecesof pipe to the free ends of the pipe conduits also consisting ofnon-crosslinked plastic polymer. The body of such pipe fittings or pipeconnections oftentimes has to sustain high mechanical stresses. The pipefitting of this invention will resist such stresses sufficiently, sinceit is made from crosslinked polymer material. The pipes which are usedfor the conduits are generally made from non-crosslinked polymermaterial so that these can be readily attached to the non-crosslinkedpipe pieces protruding from the body of the pipe connection. Thisattachment can be done by well-known welding techniques.

The advantages of the articles made in the novel mold of this inventionjust described are achieved with any crosslinked body portion andnoncrosslinked conduits of plastic materials attached thereto. Forpractical purposes, it is preferred to employ essentially the samepolymer material for the hollow body and for the conduits attachedthereto, the only difference being that the hollow body of the combinedarticle is crosslinked, whereas the attached conduits arenon-crosslinked. If, however, it is necessary or advantageous to producea combined article from different plastic materials so that the hollowbody consists of a first crosslinked polymer and the attached conduitsconsist of a second non-crosslinked polymer, this also can be done inaccordance with this invention. Such an article is also within the scopeof this invention.

Polymers useful for the purposes of this invention both in thenon-crosslinked conduits and in the crosslinked hollow body portionsare, e.g., polymers or copolymers of 1-olefins having two to eightcarbon atoms per molecule, particularly polymers of ethylene, propylene,and butene-1, copolymers of ethylene and butene-1, ethylene andhexene-1, etc. The crosslinkable polymers have been compounded with0.3-2 weight percent, based on polymers, of a crosslinking agent, andthese polymers crosslink during the rotational molding process by whichthe combined article is produced. Among the crosslinkable polymers,high-density polyethylene having a density between 0.950-0.965 g/cc anda melt index (ASTM D-1238-65T, Condition E) of 10-30 g/10 minutes ispresently preferred. The preferred crosslinking agents are acetylenicdiperoxy hexynes having the formula ##STR1## diperoxy octynes having theformula ##STR2## and diperoxy octa-di-ynes having the formula wherein Ris a tertiary alkyl, alkyl carbonate, or benzoate radical of four totwelve carbon atoms per R radical. Representative examples for theseunsaturated peroxides are the following compounds:

2,7-Dimethyl-2,7-di(t-butylperoxy)octadiyne-3,5

2,7-Dimethyl-2,7-di(peroxy ethyl carbonate)octadiyne-3,5

3,6-Dimethyl-3,6-di(peroxy ethyl carbonate)octyne-4

3,6-Dimethyl-3,6-(t-butylperoxy)octyne-4

2,5-Dimethyl-2,5-di(peroxybenzoate)hexyne-3

2,5-Dimethyl-2,5-di(peroxy-n-propyl carbonate)hexyne-3

2,5-Dimethyl-2,5-di(peroxy isobutyl carbonate)hexyne-3

2,5-Dimethyl-2,5-di(peroxy ethyl carbonate)hexyne-3

2,5-Dimethyl-2,5-di(alpha-cumyl peroxy)hexyne-3

2,5-Dimethyl-2,5-di(peroxy beta-chloroethyl carbonate)hexyne-3

2,5-Dimethyl-2,5-di(t-butylperoxy)hexyne-3.

The non-crosslinked conduits which are connected with the crosslinkedhollow body sections preferably consist of noncrosslinked polymers whichdo not contain liquid plasticizers or processing aids such as glycerine,ethylene glycol, dibutyl phthalate, and the like which are sometimesincorporated into such thermoplastic materials. These agents have thetendency to migrate towards the surface of the non-crosslinked conduits,which can result in a poor connecting strength between thenoncrosslinked conduits and the crosslinked hollow body section.

Both the crosslinked thermoplastic polymer and the noncrosslinkedthermoplastic polymer can contain further additives such as pigments,filler, antioxidants, and other stabilizers.

In accordance with a further embodiment with respect to the novel moldof this invention, there is provided a process for making a moldedplastic article consisting essentially of a hollow body of crosslinkedthermoplastic polymer and at least one conduit of non-crosslinkedthermoplastic polymer attached thereto. This process for using the moldof this invention comprises the steps of:

introducing a crosslinkable polymer into a rotational mold comprising ahot section and at least one cool section,

placing at least one conduit of non-crosslinked plastic material intosaid mold in such a manner

that a section of said conduit is located inside of said hot section ofsaid mold, during the rotational molding process,

that the crosslinkable polymer fuses and is crosslinked around and incontact with said section,

that the remainder of said conduit extends into said cool section ofsaid mold to avoid deformation of said conduits during the rotationalmolding process, heating and rotating said mold,

thereby fusing and crosslinking said crosslinkable polymer to form alayer of crosslinked polymer on the walls of said mold and a tightwelding-type connection between said layer and said section of saidconduit of non-crosslinked polymer, thus forming a composite plasticarticle, and

removing said composite plastic article from said mold.

The terms "hot section" and "cool section" of the mold are to beunderstood in a relative manner: The hot section of the mold during theheating step becomes sufficiently hot inside to fuse, mold, andcrosslink the crosslinkable thermoplastic material. The cool section ofthe mold, however, does not become hot enough to deform the remainder ofthe conduit extending into the cool section.

The insulation of the cool section of the mold can be done in variousways depending both upon the nature of the polymer and conduit involvedand the process conditions. For example, this section of the conduit tobe bonded to the cross-linked polymer body can be insulated bysurrounding this conduit with a heat-insulating material such as batt offiber glass, asbestos, or glass wool optionally covered with metal foil.

In accordance with still another embodiment of this invention there isprovided a new mold. This mold comprises a mold body section and meansfor attaching a conduit to said mold body section and for heatinsulating a first portion of said conduit. The mold body section isarranged in an apparatus for rotationally molding a plastic article insuch a manner that it can be rotated and heated such as to fusethermoplastic material contacting the inside surface of the mold bodysection. The mold body section has at least one opening. The means ofattaching and heat insulating a conduit are attached to the mold bodysection so that a second portion of said conduit protrudes through saidopening into said mold body section whereas said first portion is heldand heat insulated by said means.

Preferably the means for attaching and heat insulating the conduit is aninsulating cap attached to said mold body section and covering saidopening. More than one of such means are generally present, and in thecase of pipe connections there are at least two, in the case of pipetees there are three of such means or caps attached to the mold bodysection.

Further embodiments of this invention will become apparent from thefollowing description of the drawing and the examples.

The perspective view of FIG. 1 shows a mold opened after a finishedrotational molding process, whereas FIG. 2 shows a cross-section throughthe mold at the end of this process along the dividing plane of the twomold halves. The mold consists of two mold halves 1(a) and 1(b), whichare connected by hinges 2(a) and 2(b). The mold essentially has theshape to produce a tee-shaped pipe fitting. Connected to the mold pieces1(a) and 1(b) are six ring halves 4(a), 4(b), 4(c), 4(d), 4(e), and4(f). These ring halves are attached to the mold sections 1(a) and 1(b),which consists of a metal, for instance, aluminum or steel, by way ofhalf-ring-shaped brackets 5(a), 5(b), 5(c), 5(d), 5(e), and 5(f),respectively, which are bolted to the mold half as well as to the ringhalves. Attached to the ring halves 4(a) through 4(f) are sixcylindrically shaped half caps 6(a) through 6(f). These half caps areattached to the half rings 4(a) through 4(f) again by half-ring-shapedbrackets 7(a) through 7(f), which are bolted to the ring as well as tothe insulating half cap.

The half rings 4(a) through 4(f) consist essentially of a first section41(a) through 41(f) having a smaller internal diameter and a secondsection 42(a) through 42(f) having a larger internal configuration. Thesections 41(a) through 41(f) contact and hold pieces of pipe ofnon-crosslinked thermoplastic material 8(a) and 8(b). The sections 42(a)through 42(f) form with the pipe pieces 8(a) and 8(b) ring-shaped airgaps 43(a) through 43(f). The half caps 6(a) through 6(f) are made frominsulating material, e.g., glass fiber material, and surround theexternal ends of the plastic pipe pieces 8(a), 8(b), and 8(c). Theinternal ends of these plastic pipe pieces 8(a), 8(b), and 8(c) protrudefrom the rings 4(a) through 4(f) into the hot portion of the mold halves1(a), 1(b). In the embodiment shown in FIGS. 1 and 2, the pieces 8(a),8(b ), and 8(c) of the plastic pipe do not touch the hot section of themold 1(a) and 1(b). During the rotational molding process, crosslinkablepolymer material which is present in the mold is pulverized orgranulated form forms a tee-shaped body 9 on the inner surfaces of themold sections 1(a) and 1(b). The heat, which is externally applied tothe mold, does not reach the pipe pieces 8(a), 8(b), and 8(c) because ofthe insulating half caps 6(a) through 6(f) as well as because of the airchannels 43(a) through 43(f). These elements are the cool section of themold. Thus, the external sections of the plastic pipe pieces 8(a), 8(b),and 8(c) are prevented from softening sufficiently to collapse. The endsof the plastic pipe pieces 8(a), 8(b), and 8(c) protruding into theinterior of the hot section of the mold 1(a) and 1(b) are softenedsomewhat. This causes these ends to flare out somewhat, as shown by theflared ends 81(a) and 81(b) of the plastic pipe pieces in FIG. 2. Boththis flaring outwardly and the building up of a rotationally molded wallof the body piece 9 between the flaring end 81(b) and the neighboringwalls of the hot section of the mold 1(a) and 1(b) generate a verystrong and tight connection between the non-crosslinked pipe pieces8(a), 8(b), and 8(c) and the crosslinked body piece 9.

FIG. 3 shows a section of a mold similar to that shown in FIG. 2.However, this mold shows a hot mold section 1(a), a ring half 4(a), andan insulating half cap 6(a) (cool mold section), all having the sameinternal diameter, which in turn is the same as the external diameter ofthe pipe piece 8(a) of non-crosslinked thermoplastic material. Thesection 9 formed during the rotational molding process is thus connectedwith the pipe piece 8(a) in a butt-type joint. The combinedthermoplastic article has the same advantages of comprising a verysturdy body section 9 of crosslinked thermoplastic material and a pieceattached thereto consisting of non-crosslinked thermoplastic material,which can be easily connected to other units by conventional techniques,such as welding.

EXAMPLE

A rotational mold for a pipe of 4-inch (10.2 cm) external diameterhaving a length of about 24 inches (62.4 cm) was used. A pipe sectionabout 4 inches (10.2 cm) long was attached to each end of the mold. Thepipe pieces had an external diameter of 2.371 inches (3.5 cm). Thesections were held in place with a split ring clamp. The pipe sectionshad been extruded from a carbon black-filled copolymer of about 95weight percent ethylene and 5 weight percent butene-1. The melt flow inaccordance with ASTM D-1238-65 T, Condition F, of the polymer was 1.5gm/10 min., the density of the polymer in accordance with ASTM D-1505-67was 0.955. The plastic used was the plastic commercially available underthe trademark Marlex M-7000 from Phillips Petroleum Company.

The mold was charged with 2-1/4 pounds (1,013 grams) of high densitypolyethylene having a melt index in accordance with ASTM MethodD-1238-65T, Condition E, of 30 gm/10 min., and a density of 0.960 g/cc.The polymer contained about 0.75 weight percent of2,5-dimethyl-2,5-di(tert-butylperoxy)hexyne-3 as a crosslinking agent.The charge was in powder form obtained from grinding pellets to a sizeof about 35 mesh. The material filled into the mold is commerciallyavailable as Marlex resin CL-100 from Phillips Petroleum Company.

The pipe inserts were internally supported and plugged by a fiberglassplug. The external surfaces of the pipe were protected by a layer offiberglass insulation wrapped in aluminum foil around these endsections.

The thus prepared mold was placed into a rotational molding apparatus(800-64, McNeil three station molding apparatus) and rotated at 8 rpm onthe major axis and 9 rpm on the minor axis at a temperature of about600° F. (315.5° C.) for 15 minutes. The mold was then cooled to belowthe solidification temperature of the polymer by spraying water onto themold while continuing the rotation. Then the mold was opened and thecompleted piece has a uniform shape and was properly dimensioned forinstallation in a water system. The internal end of the pipe ends whichhad been inserted into the mold had belled out slightly and formed atight seal with the rotationally molded body.

The pipe connection was subjected to a quick burst test. It was foundthat the pipe failed before the joint failed.

Reasonable variations and modifications which will be apparent to thoseskilled in the art can be made in this invention without departing fromthe spirit and scope thereof.

We claim:
 1. A mold useful for rotational molding of thermoplasticarticles comprising:(a) a mold body section which can be rotated andheated such as to fuse thermoplastic material contacting the insidesurface of the mold body section and which mold body section has atleast one opening, and (b) means for attaching at least one conduit tosaid mold body section at said one opening and for heat-insulating afirst portion of said conduit so that a second portion of said conduitof material attached by said means to said mold body section reachesinto said heated mold body section and the first portion of said conduitis not sufficiently heated during a rotational molding process to deformsaid conduit.
 2. A mold in accordance with claim 1 wherein:said moldbody section consists essentially of thermally conductive material forheating the inside of the mold body section by contacting the outside ofthe mold body section with a heating fluid and wherein said means forattaching and heat-insulating a conduit consists essentially of aheat-insulating cap which is attached to said mold body section coveringsaid opening and to which said conduit can be attached.
 3. A mold inaccordance with claim 1 wherein said mold body section has at least onecylindrical opening, wherein said attaching means comprise:(a) a ringattached to said mold and being shaped to hold said conduit having afirst and a second portion, said first portion protruding through saidcylindrical hole into said mold body section, and (b) a cap ofheat-insulating material attached to said ring and surrounding thesecond portion of said conduit.
 4. A mold in accordance with claim 3wherein said ring has a first and a second section, wherein said firstsection faces the mold body section and contacts said conduit, whereinsaid second section of said ring faces the insulating cap and has alarger internal diameter than the first section, such as to form a ringgap between the ring, the conduit, and the insulating cap.